The electrophoretic effect is well known and has been the subject of many prior art patents and articles describing the effect. As described and discussed in the prior art, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) implement the electrophoretic effect to produce desired images. In prior art EPIDs, colored particles, which are charged either positively or negatively, are suspended in a dielectric fluid medium that is either clear or of a color which optically contrasts with the particles. The suspension is injected into a cell comprising two parallel screen electrodes, at least one of which is transparent. The colored particles are caused to migrate to, and impinge upon, one of the screen electrodes under the application of an electric field, thereby displacing the fluid medium at that electrode creating the desired image. When the polarity of the field is reversed, the colored particles migrate to the opposite screen electrode.
For suitable examples of such devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,732,830 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS and issued to Frank J. DiSanto et al. on Mar. 22, 1988. In this patent, there is disclosed an electrophoretic display apparatus which includes a planar transparent member having disposed thereon a plurality of vertically extending, electrically conductive lines defining a grid. A plurality of horizontally extending electrically conductive cathode lines are disposed on top of the vertical lines but are insulated therefrom by a thin insulating layer, thereby forming an XY matrix of electrodes. A conductive plate or anode is spaced above the line pattern and disposed therebetween in an electrophoretic dispersion of yellow submicron pigment particles in a dark colored suspension medium. The particles are transportable within the medium.
As earlier stated, the dielectric suspension consists of submicron particles of a suitable pigment element suspended in a fluid. Each of these particles is encapsulated by means of a charge control and wetting agent which interacts chemically with the particle to enable the particle to acquire an electrical charge. Providing the particles with an electrical charge is important in minimizing particle flocculation when the particles are suspended in the suspension medium and in producing electrophoretic motion.
Up until now, high density chlorinated solvents, such as carbon tetrachloride and tetrachloroethylene and the like, have been used to prepare pigment suspensions for EPIDs and the like. For example, in U.S. Pat. No. 4,655,897 entitled "ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS" issued to Frank J. DiSanto et al. on Apr. 7, 1987, tetrachloroethylene is used as a solvent in the preparation of a pigment suspension for EPIDs.
High density chlorinated solvents, in combination with low density solvents, allow density balancing between the EPID liquid medium and the medium's suspended image forming particles. Balancing is important as it allows a uniformly tinted image to be produced over the entire display panel.
In recent years, however, chlorinated solvents have been targeted by regulators because of their potentially harmful environmental effects and toxicity. As a result of this, it is anticipated that the use of chlorinated solvents will be subject to strict restrictions in the future. Hence, the search for alternative solvents, which will replace chlorinated solvents, is of great importance.
Liquids that are completely fluorinated are dense and colorless, and have the tendency to be relatively inert. A combination of a dense fluorinated solvent along with a less dense hydrocarbon liquid, produce a stronger solvent system which is suitable for making dielectric fluid suspensions for EPIDs and similar devices.
It is well known that most fluorinated materials have low surface tension. This characteristic causes fluorinated liquids to be difficult to use in applications involving the wetting of a hydrocarbon substrate, such as, with organic or inorganic pigment surfaces. Thus, agglomerations of pigment particles are difficult to disperse in a fluorinated medium. As such, a stable suspension cannot be formed and the performance of the resulting device is unacceptable.
One method for improving a dispersion's stability is to introduce a surfactant which preferentially adsorbs onto the pigment surface to lower the interfacial tension between the pigment surface and the medium for wetting, and provides a certain electrostatic or steric repulsion for long term stability.
Stabilization of a dielectric suspension depends upon the solubility of the surfactant in the medium and the interaction between the surfactant and the pigment surface. Consequently, selection of the correct surfactant for a particular solvent combination is critical in making a stable dielectric suspension for an EPID or like device.
It is, therefore, an object of the present invention to provide an environmentally acceptable dielectric suspension composition comprising a fluorinated solvent medium which exhibits excellent optical contrast, high electrophoretic speed, long operating lifetime, and good suspension stability in an EPID or other like device.
It is a further object of the present invention to provide a method for making an environmentally acceptable fluorinated dielectric suspension.